1. Field of the Invention
The invention relates to optical transmitters and receivers, and in particular to electronic circuits that generates a nonlinear signal with adjustable phase and frequency dependence that can be used to cancel the distortion of electronic or electro-optical devices, such as semiconductor lasers or photodiodes, when connected either at the input or at the output of the device that needs to be linearized.
2. Description of the Related Art
Analog electro-optical devices such as lasers and photodiodes used in CATV and FTTH systems require a high degree of linearity. Pre-distortion and post-distortion circuits are commonly employed to correct the nonlinear behavior of opto-electronic and electronic devices by adding a nonlinear signal that cancels the undesired distortion introduced by the non-linear laser or photodiode. In order to achieve good cancellation, the added distortion needs to match in phase, magnitude and frequency content that is introduced by the nonlinear device. Among pre-distortion and post-distortion techniques, in-line circuits, such as those described in U.S. Pat. No. 5,798,854, generally provide sufficient degree of linearization at a low cost. However, while pre- and post-distortion circuits described in the prior art are able to achieve sufficient degree of pre-distortion for some applications, they do not provide enough flexibility to create different frequency and phase characteristics. For example, RF amplifiers operated with low supply voltage for lower power dissipation require a large amount of distortion
Some distortion circuits disclosed in U.S. Pat. No. 5,798,854 can introduce high RF loss in the transmission path and create undesired reflections when large amounts of distortion need to be generated. There are requirements that will enable large distortion with low loss and low reflection that are not satisfied by the prior art design. Moreover, some distortion circuits, such as those disclosed in U.S. Pat. No. 5,798,854 that are designed to correct second order distortion also produce undesired third order distortion, and such designs do not allow tuning of both second and third order distortion simultaneously.